The radiation power of an antenna for a radio telephone, generally receiver telephone, can in a manner known in the an be increased by placing between the telephone and the antenna a radio frequency power amplifier, a so-called "booster", or "booster amplifier". In practice also the amplification of the receiver direction must be arranged within the booster. The booster increases the radiation power of the antenna by increasing the input current of the antenna, so that the radiation power will be increased in proportion to the square of the power. For cost reasons, endeavours have been made to make the design of the booster as simple as possible, e.g. so that only the indispensable power amplifiers are provided therein.
A booster amplifier could, in principle, be connected to a digital radio telephone, preferably a digital TDMA radio telephone in a way similar to that used in analog telephones, but in practice an implementation like that would be very complicated and costly. In digital radio TDMA telephones transmissions are carried out in bursts, whereby a signal to be transmitted consists of consecutive pulses. The rise and decay of a transmission pulse should not be step-like, but the rise and decay thereof should be controlled so that the transmission spectra would not spread too much. In order to maintain the rise and decay times as short as possible the cos.sup.2 -waveform has frequently been considered to be an appropriate rising and decaying pulse shape. The timing of the transmission pulse is moreover expected to be independent of the power level of the transmitter. By means of power control, the interference level of the network is reduced and the power level of telephone is decreased, said control being based on measurements made by the base station e.g. in the ((Global System Mobile) GSM system. The control range in said system is 30 dB, made out of 16 power levels within the range +43 dBm to +13 dBm.
A GSM system radio telephone is used as an example below, but the description is also applicable for other digital systems in which the shape of the pulse rise and decay is cos.sup.2 or equivalent. The transmitter of a digital radio telephone known in the art comprises a voltage-controlled RF power amplifier controlled by a control amplifier conforming to the output power of the power amplifier and the control logic. A typical block diagram of a GSM radio telephone transmitter is presented in FIG. 1. The input square wave pulses P.sub.in are therein amplified as desired and, as regards their rise and decay, converted into cos.sup.2 shape to form power output pulses P.sub.out in a multiple stage power amplifier 1. The output power is measured with the aid of a directional coupler 2 and with a power sensor 3 providing voltage V.sub.1 comparable to the power. Said voltage is carried to a control amplifier 4, which can be an operational amplifier; a control pulse TXC from a D/A converter enters a second input of the control amplifier. With the aid of the control pulse, the amplitude of a transmission pulse P.sub.out at a desired power and the waveforms of the rising and decaying edges are formed. If the voltage V.sub.1 is higher than voltage TXC, the output voltage of the power amplifier 4 will be reduced, thus resulting in the selected output power. Thus, endeavours are made to maintain with the transmission pulse the control loop 1, 2, 3, 5 in the same shape as the control pulse.
The Finnish patent application FI-896266 (equivalent U.S. Pat. No. 5,101,175 and EP patent application EP-A434 294) discloses a power control method based on two control signals for a digital radio telephone, in which method a square wave pulse is summed with the control voltage V2 (FIG. 1) of a power amplifier, said pulse starting and ending substantially at the same instant of time as a control pulse entering the control amplifier 4. In a most preferred embodiment, the square wave pulse is turned off when the control circuit of the power amplifier 1 starts to operate. In said application, a basic feature of the invention is that with a square wave pulse, the value of the control voltage V2 of the power amplifier 1 is rapidly raised at the start of the control pulse TXC close to the threshold level at which the power amplifier 1 starts to operate. Hereby, the control circuit 5 is enabled to operate as soon as power is being transmitted from the power amplifier 1. No significant delay is produced in such instance, neither resembles a rise in the output power P.sub.out a step at the beginning of the pulse. This is the method in which a good cos.sup.2 shape of a transmission pulse P.sub.out of the power amplifier 1 can be obtained right at the beginning of the pulse, and a follow-up of the TXC control pulse almost without any delays. The method introduced here is useful on the power levels required.
In designs of the type known in the art the timing information, on the basis of which either the reception or transmission is amplified, is produced from an incoming RF signal. When an RF signal enters the booster, it must respond quickly in order to raise the power level, as was stated above. Although, as according to Finnish patent application FI-896266, a good cos.sup.2 shape of the transmission pulse P.sub.out can be provided right at the beginning of a pulse and a follow-up of a TXC control pulse with almost no delays, the transmission spectrum will nevertheless spread to some extent.
In TDMA based radio telephone systems, a significant problem in implementing a separate high-frequency data amplifier (called an RF booster below) is how to enable the RF booster to operate entirely in synchronization with the radio telephone when the design is required to be simple, reliable, inexpensive, and applicable in a simple manner for a variety of diverse radio telephone systems, such as DAMPS, JDC and GSM.